1. Technical Field
This invention relates generally to an apparatus and method for fabricating semiconductor thin films. More specifically, the invention relates to a method and apparatus for effectively reducing process solid buildup during a low pressure chemical vapor deposition film fabrication process.
2. Description of the Prior Art
The following four documents relate to methods dealing with hot and cold traps for isolating source materials from a reaction chamber.
U.S. Pat. No. 4,168,741 issued Sep. 25, 1979 to K. Degenhardt et al., shows an apparatus for ammonia feed in copying equipment.
U.S. Pat. No. 5,015,503 issued May 14, 1991 to R. D. Varrin, Jr. et al., discloses an apparatus for producing compound semiconductor thin films. The apparatus includes hot and cold traps for isolating source materials from the reaction chamber and to provide for controlled delivery of the species. The hot and cold traps communicate with the reaction chamber through hot and cold legs to establish a closed loop circulating flow.
U.S. Pat. No. 5,250,323 issued Oct. 5, 1993 to S. Miyazaki discloses a chemical vapor growth apparatus having an exhaust device including trap. The patent includes a trap for absorbing the source gas remaining in the source gas inlet and the gas flow-rate controller, provided in the first exhaust.
U.S. Pat. No. 5,261,963 issued Nov. 16, 1993 to W. Basta et al., discloses a CVD apparatus comprising exhaust vapor condensation means. The apparatus comprises a reactor having a chamber with a coating region for coating a substrate and an exhaust region. A condensing assembly is disposed in the exhaust region for condensing excess, unreacted gaseous reactant from the spent gas before entry into the outlet. The condensing assembly includes a high surface area, apertured structure disposed in the exhaust region where the temperature of the spent gas stream in sufficiently reduced to condense excess, unreacted gaseous reactant therefrom.
Low pressure chemical vapor deposition (LPCVD) is a well known method for depositing and forming a protective thin film on substrates. Typically, the substrates are loaded into a reaction chamber, heated to a suitable reaction temperature, and exposed in the reactor to one or more elevated temperature gaseous reactants that react with the substrate surfaces to form a coating or layer thereon. The LPCVD deposited coating or layer can be reacted with the substrate by suitable heating in the reactor to form a protective diffusion coating thereon.
An ammonium chloride by-product is produced during a silicon nitride process within a LPCVD reactor while forming a thin silicon nitride layer on a semiconductor substrate. The ammonium chloride is produced in a vapor phase and evacuated through the exhaust port while rapidly crystallizing into a vapor/crystal mix . The vapor/crystal mix is drawn into a cold trap.
By definition, a cold trap functions as a cryopump. System pressure is reduced by a low temperature condensation of vapors in the vacuum system. When a surface is introduced into the system at a temperature low enough to condense the vapor present, the resultant pressure in the system will be that of the vapor pressure of the condensate. As the condensate solidifies, a slight decrease in the temperature of the solid will usually result in a sharp reduction in the vapor pressure. Cryopumping is based on this principle. In some cases traps are used to keep pump fluid vapors (oil or mercury) out of the vacuum circuit or out of other pumps by condensing or freezing these vapors. In other cases, the primary function of a trap is to cut down on the pump-time by pumping various condensable vapors.
Cold traps used in conventional LPCVDs collects and restrains the NH.sub.4 Cl solids from entering the vacuum pump. This prevents solids from damaging the pump while permitting evacuation of the vapors. As the build-up of NH.sub.4 Cl solids increase within the cold trap, system pressure becomes increasingly difficult to control. Resistance to the evacuation of vapors from the reaction chamber, caused by the increased volume of solids in the cold trap circuit, requires shutting down the LPCVD process for servicing. Maintenance personnel must disassemble and clean the vacuum circuit, hence, reducing equipment process utilization.
FIGS. 1a through 1c schematically illustrate several embodiments of exhaust circuits of the prior art. Each embodiment includes a LPCVD reactor 10 having an exhaust outlet 14, a pressure sensor 15, a first valve 23, a cold trap 11 and a vacuum pump 21 all disposed in series in the order described. A second valve 24 is positioned to give an alternate path for bypassing first valve 23.
During LPCVD preparation, all valves are closed, a semiconductor substrate and a source material, not shown, are placed and sealed inside the reactor tube 20. At the start of the deposition cycle, the second valve 24 is opened to restrict the rush of air from the reactor tube thereby preventing disturbance of NH.sub.4 Cl solids, within the exhaust circuit, from being drawn into the vacuum pump 21. As the chamber pressure is reduced to a preset maximum, the first valve 23 is opened to expedite the pressure pump down to a given process pressure. The source material is heated to a preset vaporizing temperature. The vapor surrounds and reacts with the semiconductor substrate forming a nitride protective film thereon while yielding a NH.sub.4 Cl byproduct vapor. The byproduct vapors are evacuated and begin to crystallize as exiting into the exhaust outlet 14. As the vapor/crystal mix is pulled into the exhaust circuit, passing through the opened first valve and into cold trap 11. The crystals are trapped and held back permitting only the vapors to be pulled out and vented by pump 21. The temperature of the cold trap 11 is lowered by circulating process cooling water into an attached cold plate having extended conductive surfaces to help solidify and retain the NH.sub.4 Cl crystals from being ingested by the pump 21.
Typically, the NH.sub.4 Cl by-product will crystallize at sufficiently cool locations in the reactor exhaust circuit and eventually plug or clog the exhaust circuit. The problem of exhaust clogging or plugging affects process pressure control resulting in shut-down of the LPCVD apparatus. Preventive maintenance requires lengthy disassembly and cleaning procedures reflecting in increased manufacturing cost due to lower equipment utilization.